I've had this for a while. Putting it up here in case this is helpful to others. To use, create "htdocs" in "FileDirectory default". Web-wise, "htdocs" is known as "/static". I serve my CSS and JS files from there.

WAComancheAdaptor subclass: #WAComancheStaticAdaptor
    instanceVariableNames: ''
    classVariableNames: ''
    poolDictionaries: ''
    category: 'Seaside-Adaptors-Comanche'

WAComancheStaticAdaptor>>createService
    | contentPath dirPath  svc ma |

    contentPath := 'htdocs'.
    dirPath := FileDirectory default fullNameFor: contentPath.
    svc := (HttpService on: self port)
        name: 'seaside-' , self port greaseString.
    ma := ModuleAssembly core.
    ma alias: '/static' to: [
        ma serverRoot: dirPath.
        ma documentRoot: dirPath.
        ma directoryIndex: 'index.html index.htm'.
        ma serveFiles ].
    ma addPlug: self.
    svc plug: ma rootModule.
    ^ svc